So this should answer the question, but you can use which as long as it is supported by the browsers independently from W3C specs.

Scott showed that bitmap of 1, 2, 4

Which is also in the current spec under buttons

The buttons being pressed when the mouse event was fired: Left button=1, Right button=2, Middle (wheel) button=4, 4th button (typically, "Browser Back" button)=8, 5th button (typically, "Browser Forward" button)=16. If two or more buttons are pressed, returns the logical sum of the values. E.g., if Left button and Right button are pressed, returns 3 (=1 | 2).

Maybe interesting for some drawing applications, with multiple gestures types..

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